Electron accelerator of the microwave type



Oct. 3, 1950 w, c, BROWN 2,524,252

ELECTRON ACCELERATOR 0F THE MICROWAVE TYPE Filed Feb. 5,4 1948 Patentedoct. 3, 1950 ELECTRON ACCELERATOR OF THE MICROWAVE TYPE William C.Brown, Lincoln,` Mass., assignor to Raytheon Manufacturing Campana,Newton, Mass., a corporation of Delaware Application February 5, 1948,Serial No. 6,416

8 Claims. l

This invention relates to electron accelerators, and more particularlyto microwave devices for so accelerating electrons as to cause the sameto attain velocities representing energies measuring in the millions ofelectron-volts.

Microwave devices for this general purpose have heretofore not heen verysatisfactory due to the difficulty of maintaining the electrons in stepwith the microwave energy utilized for the acceleration.

The phase velocity of microwave energy the usual microwave device is inexcess of the speed of light, and inasmuch as it is not possible to movecharges at such speeds, the electrons soon fall out of step with theaccelerating energy and only very limited electron velocities result.

It is, therefore, among the objects of the present invention to providea microwave device incorporating means for reducing the phase velocityof the energy propagated therethrough to less than the speed of light,whereby electrons `projected therein are enabledto keep in step with anygiven phase of such energy and, so, become continually accelerated tothe desired velocities.

It is another object of the present invention to provide a microwavedevice in which the electrons traveling therethrough are subjected tothe ac celerating influence of microwave energy, the

phase velocity of which changes along the length of the device, wherebyas the electrons acquire greater and greater velocity, they are affectedby energy of continually increasing phase velocity.

These, and other Objects of the present invention, which will becomemore apparent as the den tailed description thereof progresses, areattained, briefly, in the following manner:

The phase velocity op of microwave energy propagated down a wave guidemay be expressed by the equation:

U11- where w is the angular frequency of the energy, and o is the phaseconstant thereof;

Now, inasmuch as, for any particular' angular frequency, the phaseconstant is determined by the physical parameters of the wave guide, the

phase constant of the energy of said particular angular frequency may beconsidered as the phase `constant of said wave guide.

If, in Equation l, the phase constant is given the phase velocity of thewave guide under consideration As stated in earlier portions of thisspecification, it is desired, iirst, to reduce the phase velocity tosomething lessV than the speed of light and, then, to increase saidphase velocity along the length of the guideto bring about an irl-phaserelationship between the velocity of the accelerating electrons and anygiven phase of the microwave energy. In other words, it is desired that:

where c is the velocity of light, and a: is a nite value which decreasesalong the length of the guide in a direction receding from the pointofcoupling between the microwave generator and said guide.

Transposing the terms of Equation 2, the phase constant may beexpressed:

Means are provided, adjacent the point of coupling between the energygenerator and the wave guide, for introducing electrons into the latter,the energy propagated through the Wave guide continually acceleratingsaid electrons to give the saine velocities corresponding to millions ofelectron-volts.` A target at the far end of the wave guide may bebombarded by the accelerated electrons,possibly, to emit X-rays, and thelike, or, possibly for useinerely as a source of high voltage.

Inthe accompanying specication there shall be described, andin theannexed drawing shown, an illustrative embodiment of the electronaccelerator of the present invention. It is, however, to` be clearlyunderstood that the present invention is not to belirnited to thedetails `herein `shown and described forpurposes of illustration only,inasmuch as changes therein may be made without the exercise ofinvention, and within the `true spirit and scope of the claims hereto'appended.

In said drawing, the single` gure is a vertical sectional view takensusbtantially through the center of a microwave device made inaccordance with the principles of the present invention.

Referring now more in detail to the aforesaid illustrative embodiment ofthe present invention, with particular reference to the drawingillustrating the same, the numeral Il] generally designates an electronaccelerator [comprising a cathode structure I I, an anode structure I2spaced from and surrounding said cathode structure, means I3 forestablishing a magnetic field in the space between said cathode andanode structures in a direction transverse to the electron paththerebetween, the structures I I, I2 and I3 constituting a microwavegenerator of the magnetron type, and means I4, receptive of the energygenerated by said microwave generator, for accelerating electronsprojected `therein as will hereinafter be more fully described.

The cathode structure II includes a sleeve I5 made, preferably, ofnickel, and having a portion thereof coated, as at I6, withelectron-emissive material, for example, of the alkaline earth metaloxide type, the coated portion being bounded by anges I'I constitutingshields for preventing passage of the emitted electrons longitudinallyof the sleeve I5.

The upper end of the sleeve I5 is closed by a disk I8 to the innersurface of which one end of a cathode heater I9 is xed, said cathodeheater passing downwardly through the sleeve I5 and being connected atits lower end, as at 2t, to a lead-in conductor 2l, the latter, in turn,passing downwardly and out of the sleeve I5 by way of a glass seal 22.Heating current may be supplied to the heater I9 by means of a voltagesource 23 connected between the sleeve I5 and the conductor 2l.

The anode structure I2 includes a cylindrical member 211 made,preferably, of copper, and provided with a plurality ofinwardly-directed, radial vanes 25, the inner ends of said TJanes beingspaced from the cathode sleeve I5 and providing electron-receiving faces25 substantially coextensive with the coated portion I6 of said cathodesleeve. with that portion of the cylindrical member 24 lyingtherebetween, constitute a cavity resonator adapted, when suitablyexcited, to generate electrical oscillations of a frequency which is afunction of the geometry of the elements making up the same. Preferably,vthe device of the present invention is so dimensioned as to generateenergy in the microwave region of the spectrum, for example, of a wavelength of 10 centimeters or less.

In order to suppress spurious oscillations or provide adequate modeseparation and, thus, increase the efficiency of the device, alternatevanes 25 are electrically interconnected, at the upper edges thereof, bya low impedance conductor 21 and, at the lower edges thereof, by asimilar conductor 28. Intervening vanes 25 are electricallyinterconnected, at the upper edges thereof, by a low impedance,dish-shaped member 29 having a central opening, 3! and, at the loweredges thereof, by a low impedance conductor 3|.

The lower end of the cylindrical member 24 is closed by an end plate 32and the upper end thereof is closed by a glass dome 32.

In order to bring the cathode structure II to the outside of the anodestructure I2, the' end plate 32 is provided with a central aperture inwhich a sleeve is xed, the lower end of said sleeve being fastened to aglass sleeve 34 which, in turn, is fastened to a ferrule 35 carried bythe cathode sleeve I5. l

Each pair of adjacent vanes 25, together The magnetic field establishingmeans I3 includes dish-like pole pieces 36 and 37 spaced from the Vanes25 at the upper and lower edges thereof, said pole pieces having flangeportions 38 and 39 extending through the cylindrical member 24 of theanode structure I2 and engaging, for example, a horseshoe magnet 50,whereby a magnetic field is set up in the interelectrode region of thedevice, in a direction perpendicular to the electron path between thecathode and anode structures.

When a device, such as has thus far been described, is supplied withheating current, as from the voltage source 23, and anode voltage, asfrom a voltage source 4I, one terminal of which is connected to thecathode sleeve I5 and the other terminal of which is grounded, as is theanode structure I2 itself, electrical oscillations are generated in thecavity resonators thereof in a manner well known to those skilled in theart.

In the present invention, the dish-shaped conductor 29 serves twopurposes. It constitutes one of the elements of the mode-suppressing orseparating arrangement described in earlier portions of thisspecification, and it functions as a coupling member to lead the energygenerated in the cavity resonators of the device out of the same. Asshown herein, said conductor 29 has affixed thereto a wave guide, forexample, a spiral rod 42, the turns of which are increasingly spacedfrom each other as it recedes from its point of contact with thecoupling conductor 29. Ordinarily, the phase velocity of anelectromagnetic wave traveling down a wave guide is in excess of thespeed of light. However, this velocity can be reduced to less than thespeed of light, for example, by causing the wave to travel over a path,the length of which is greater than the linear dimension of the guide.In such case, the phase velocity along the extended path will still begreater than the speed of light, but by selecting a proper ratio ofdistance along the extended path to the distance along the vlineardimension of the guide, the velocity along said linear dimension may bemade less than the speed of light.

Consider the spiral member 42 and neglect, for the moment, the unequalspacing of the turns thereof. When this wave guide is excited, the wavewill move along a spiral path with a phase velocity (Equation 1) v andthis velocity will exceed the speed of light;

but, the distance covered by any given phase per unit time along thelinear dimension represented by the axis of the guide will be much lessthan the distance covered by said phase in the same unit time along saidspiral path, and if the diameter of the member #l2 is suciently great,the linear phase velocity will be less than the speed of light. l

The present invention takes advantages of this phenomenon by providingthe outer surface of when a point immediately beyond the ,entrance tothe guide is at a positive potential with respect to a point at saidentrance become accelerated, and, if the guide is properly constructedto cause the accelerating phase to move along the guide at increasingAvelocities, said electrons remain in step with said accelerating phaseand, eventually, attain tremendous velocities.

As indicated above (Equation 2), it is desired that the phase velocitybe made equal to something less than the speed of light, in otherwords,`

to c-.r or that the phase constant of `the-guide be made equal (Equation3) to `C IIJ uHowever, to satisfy the condition described in theprevious paragraph, the phase constant must decrease linearly along theguide. This lis accomplished by increasing the spacing between adjacentturns of the guide herein shown, in other words, decreasing the finitevalue of x in the expression c-. The linear phase velocitt7 increasesbecause by increasing turn spacing, any

given phase travels a greater linear distance per unit time.

The device of the present invention is completed by supporting a target44 adjacent the outer end of the wave guide 42, which target may besealed through the glass dome 32 and groundif ed through a load 45. Asthe accelerated electrons pile up on the target, the potential of saidtarget increases with respect to ground. If desired, the target 44 maybe replaced by one which, when bombarded by high-speed electrons, emits,QS-rays or the like. H i

Thus, there is provided asirnple electron accelerator or voltagegenerator utilizing microwave energy for the electron acceleration, saiddevice incorporating a wave guide in which the phase velocity of theenergy propagated therethrough is reduced to less than the speed oflight, whereby electrons projected therein are enabled to keep in stepwith any given accelerating phase of the propagated energy and in whichthe phase veloc- "7 where w is the angular velocity of said energy, c isthe velocity of light, and :r is a finite value which decreases alongthe length of said wave guide in a direction receding from the point ofcoupling between said energy-generating means and said wave guide; andmeans, adjacent said point of coupling, for introducing electrons intosaid wave guide.

2. An electron accelerator comprising: a magnetron oscillator forgenerating microwave energy of a predetermined wave length; a waveguide, coupled to said oscillator, and having a phase constant, relativeto sid energyof predetermined wave length, which is equal to C$ where. eis the angular velocity of said zenergy, c is` the velocity of light,and :r is' aiinitesvalue which decreases along the length of said waveguide in a direction receding from the pointlof coupling` between "saidoscillator and said 4wave guide; and a commonmeans, adjacent said pointof coupling, forintroducing electrons into said wave guide and into`said magnetron oscillator.

3. An electron accelerator comprising: a cathode; an anode structure,spaced from said cathode, and incorporating a plurality of cavityresonators in which microwave energy of a predetermined wave length isadapted to be generated; a wave guide, coupled to said cavityresonators, and receptive of said energy of predetermined wave length;said wave guide having a `phase constant, relative to said energy ofpredetermined wave length, which is equal to where w is the angularvelocity oi?` said energy, c` is the velocity of light, and x is afinite value which decreases along the length` of said wave guide? in adirection receding fromthe point'of coupling between saidcavityresonators and said wave guide; and means, adjacent said pointofcoupling, for introducing electrons into said wave guide. f

4. An electron accelerator comprising: a cathode; an anode structure,spaced from said cathode, and provided with a plurality ofradiallydisposed arm members; each pair 'of adjacent arm members,together with that portion of said anode structure lying therebetween,constituting a cavity resonator in which microwave energy o1 apredetermined wave length is adapted to be generated; a pair ofconductive members electrically connecting, respectively,` alternate andintervening anode arm members;v a wave guide coupled to one of saidconductive members; said wave guide having a phase constant. relative tosaid energy of predetermined wave length, which is equal to where `w isthe angular velocity of said energy, c is the velocity of light, and o:is a finite value which decreases along the length of said wave guide ina direction receding from the point of coupling between said one of saidconductive where w is the angular velocity of said energy, c is thevelocity of light, and is a nite value which decreases along the lengthof said wave guide in a direction receding from the point of couplingbetween said oscillator and said Wave guide; and means, adjacent saidpoint of coupling, for projecting electrons into the space bounded bysaid helix along the axis thereof.

6. An electron accelerator comprising: a cathode; an anode structure,spaced from said cathode,and incorporating a plurality of cavityresonators in which microwave energy of a predetermined wave length isadapted to be generated; an electrically-conductive helix coupled tosaid cavity resonators, and having successive turns increasingly spacedfrom each other along the axis thereof; said helix constituting a waveguide having a phase constant, relative to said energy of predeterminedwave length, which is equal to where .w is the angular velocity of saidenergy, c is the velocity of light, and is a nite value which decreasesalong the length of said wave guide in a direction receding from thepoint of coupling between said cavity resonators and said wave guide;and means, adjacent said point of coupling, for projecting electronsinto the space bounded by said helix along the axis thereof.

7. An electron accelerator comprising: a cathode; an anode structure,spaced from said cathode, and provided with a plurality ofradiallydisposed anode arm members; each pair of adjacent arm members,together with that portion of said anode structure lying therebetween,constituting a cavity resonator in which microwave energy of apredetermined wave length is adapted to be generated; a pair ofconductive members electrically connecting, respectively, alternate andintervening anode arm members; an electrically-conductivehelix coupledto one of said conductive members, and having successive turnsincreasingly spaced from each other along the axis thereof; said helixconstituting a wave guide having a phase constant, relative to saidenergy of predetermined wave length, which is equal to where w is theangular velocity of said energy, c is the velocity of light, and a: is anite value which decreases along the length of said Wave guide in adirection receding from the point of coupling between said one of saidconductive members and said wave guide; and means, adjacent said pointof coupling, for projecting electrons into the space bounded by saidhelix along the axis thereof.

8. A high voltage generator comprising: a primary cathode; an anode,spaced from said primary cathode, and incorporating a cavityresonator inwhich microwave energy of a predetermined wave length is adapted to begenerated; a wave guide, coupled to said cavity resonator, and adaptedto be excited by said microwave energy; said wave guide having a phaseconstant, relative to said energy of predetermined wave length, whichdecreases along the axis thereof in a direction receding from the pointof coupling between said cavity resonator and said wave guide; asecondary cathode, disposed adjacent the'relatively high phase constantend of said wave guide, and adapted to introduce electrons therein; atarget electrode, disposed adjacent the relatively low phase constantend of said wave guide, and adapted to intercept the electrons emittedby said secondary cathode and accelerated toward said target along saidwave guide; and an evacuated envelope enclosing said primary andsecondary cathodes, said anode, said wave guide, and said targetelectrode.

WILLIAM C. BROWN.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,233,126 Haef Feb. 25, 19412,300,052 Lindenblad Oct. 27, 1942 2,414,085 Hartman Jan. 14, 19472,434,508 Okress et al. Jan. 13, 1948 2,442,118 Donal, Jr., et al. May25, 1948

